Different from a traditional solar shading fashion such as a curtain, a louver and the like, a thermal response smart window produced by a thermal response material can adjust an intensity of incident light and an irradiation heat immediately without using a mechanical force, which will facilitate human's living and working greatly.
A thermal response material can be classified into three types, such as thermally light scattering material (or thermotropics), a thermochromism material (or thermochromics), and a material with both of the above two properties. A thermal light scatting material is a kind of material which can vary an optical transmittance spontaneously along with the variety of temperature, such as transforming from a transparent state to a milkiness light scatting state (transparent-cloudy transformation). A thermochromism material is a kind of material which can vary the absorption characteristics of the visible band along with the variety of temperature, and a color variety can be identified visually. The third type of material which has both of the two properties is with the two kinds of functions simultaneously, i.e. both the transparent-cloudy transformation and the color variety occur along with the variety of temperature.
The light-adjusting principle of a thermally light scattering material is as follows, above the transformation temperature (i.e. cloud point), some micro-phases, micro-crystalline or micro-areas with unmatched refractive indexes caused by a phase separation and a aggregation structure transformation, allow for an incident light scatting inside the material, showing a cloudy state. After the temperature is reduced below the cloud point, these micro-phases, micro-crystalline or micro-areas disappear gradually, and the system goes back to a homogeneous phase, showing a transparent state again. In most of the applications, the thermally light scattering polymer system needs to satisfy the following requirements: the reversible transparent-cloudy transformation occurs; the transmittance at transparent state is or more than 75%, the transmittance at cloudy state is or less than 15%; the cloudy state of the system is homogeneous, which will not cause the eyes' discomfort; the material has predetermined mechanical and dynamical properties, and good stability; the material has higher cycle life on transparent-cloudy transformation.
The known the thermally light scattering polymeric materials are classified into following types:
1) The hydrogels with lowest critical solution temperature (LCST). The hydrogels occur the reversible phase separation with water above the LCST of the hydrogels so that the hydrogel system will exhibit the light scattering state because, and the hydrogels are placed into double-glass or polymeric films and then form a hydrogel-glass with a smart thermal response (See both A, Schneider J, Patzak A. Materials for intelligent sun protecting glazing. Sol Energy Mater Sol Cells, 2000, 60: 263; Nitz P, Hartwig H. Solar control with thermotropic layers. Solar Energy, 2005, 79: 573). In actual application, the thermal response smart hydrogel-glass device produced by the hydrogel is big in volume and thick because the water content of the hydrogel is very high (about 90%) and the device needs to be specifically sealed, it will restrict the use of the hydrogel-glass device. Furthermore, the mechanical property of the hydrogel is very poor, and then it is hard to be used as thin film material and it is easily to milden and rot.
2) The thermal light scatting polymeric thin film material disclosed in U.S. Pat. No. 5,916,972 (1999) is consist of the mixture of two or more two polymers. The extent of compatibility of the polymers correlates to temperature. When the temperature is below the lowest critical solution temperature (LCST), the polymeric components are compatible and they are sample phase, and then the material is transparent; when the temperature is above the lowest critical solution temperature (LCST), the polymeric components are not incompatible and they are separated phases, and then the material is opaque. Although such thermal light scatting polymeric film does not comprises water, the organic solvents such as benzenes, halogenated hydrocarbon, and the like, which are harmful for health, need to be used. In addition, these organic solvents can not be removed completely, and then they will remain in the finished polymeric film, and will influence the use thereof.
3) U.S. Pat. No. 5,977,201 (BASF Aktiengesellschaft) disclosed a crosslinked thermal light scatting polymeric thin film. The light-adjusting property is also obtained based on the characteristic of thermal phase separation of the blended polymers. Although the finished thin film does not contain any organic solvent with increased mechanical properties, it still has some disadvantages as follows: firstly, it is need to be specifically designed and carefully prepared the polymer with specific structure, including molecular weight and distribution thereof, and the like, to achieve and control the thermal phase separation; secondly, when preparing the material, two polymers are dissolved with an organic solvent, formulated into a solution, casted onto a substrate, and removed the solvent to form the thin film, and then the formed the thin film is crosslinked by UV lights. Therefore, such method is very complex with a long preparation cycle, and it needs to use volatile organic solvent, and then it increases the cost and is adverse to environmental protection.